Not applicable.
The present invention relates to the calendering of a fibrous web.
Calendering is a method by means of which the properties, in particular the thickness profile, smoothness, gloss, surface porosity and translucence of a web-like material, such as a paper web, are sought to be generally improved. In calendering the paper web is passed into a nip which is formed between rolls pressed against each other and in which the paper web is deformed by the action of temperature, moisture and nip pressure, in which connection the physical properties of the paper web can be affected by controlling the above-mentioned parameters and the time of action. The good physical properties attained by calendering lead to better print quality, thereby bringing a competitive advantage to the manufacturer of paper.
The so-called shoe rolls known in prior art are usually hydraulically deflection-compensated, zone-controlled rolls in which the shell is supported from a non-rotating central shaft of the roll by means of hydrostatic loading arrangements, such as rows of loading shoes, which transfer the nip force acting on the shell rotating around the central shaft so as to be carried by the central shaft. The loading element is generally also divided into zones, in which connection the loading pressure can be regulated as required by profiling. The zoning in this kind of zone-controlled shoe roll may comprise individual elements in the loading arrangement, in which connection the number of zones in the roll and in the loading arrangement may exceed 60xe2x80x94as examples may be mentioned the shoe rolls marketed by Metso Paper, Inc. under the trademarks SymCD(trademark) and SymCDS(trademark), or the zoning may comprise a group of elements in the loading arrangement, in which connection the roll and the loading arrangement normally comprise eight zonesxe2x80x94as examples may be mentioned the shoe rolls marketed by Metso Paper, Inc. under the trademarks SymZ(trademark), SymZS(trademark), SymZL(trademark), and SymZLC(trademark). Extended-nip calendering accomplished by means of a shoe roll has generally been found to be good for producing low-gloss paper grades, i.e. grades having a Hunter gloss % below 40. When higher gloss is required, the nip pressure of extended-nip calendering is, however, not any more sufficient to provide gloss.
In the papermaking art, grades of ever higher quality are required today. As the running speeds required of paper machines are continuously increasing, the direction in calendering technology is more and more towards on-line solutions. When the aim is to make higher quality printing paper grades, such as, for example, SC-A and LWC-roto grades and glossy coated paper grades, a substantial problem is that this kind of grades can be produced in practice only by using, after drying a fibrous web, intermediate winding and off-line supercalenders, several of said supercalenders, usually two or three, being used side by side to meet production capacity.
Supercalendering is calendering in a calender unit in which nips are formed between a smooth-surface press roll, such as a metal roll, and a roll covered with a resilient coating, such as a paper or polymer roll. The resilient-surface roll adapts itself to the contours of the surface of paper and presses the opposite side of paper evenly against the smooth-surface press roll. Today, the supercalender typically comprises 10-12 nips and for the purpose of treating the sides of the web, the supercalender comprises a so-called reversing nip in which there are two resilient-surface rolls against each other. Linear load increases in the supercalender from the top nip to the bottom nip because of the force of gravity. This increase in load can be compensated for by using the relieving of the rolls. Supercalendering is an off- and on-line calendering method, and at the moment it provides the best paper qualities, such as, for example, WFC, LWC-roto and SC-A.
Soft calendering is calendering in a calender unit in which nips are formed between a smooth-surface press roll, such as a metal roll, and a roll covered with a resilient coating, such as a paper or polymer roll. In a soft calender, the nips are formed between separate roll pairs. In order to treat both sides of the web in the soft calender, the order of the roll pairs forming the successive nips is inverted with respect to the web so that the resilient-surface roll may be caused to work on both surfaces of the web. Soft calendering is an on- or off-line calendering method, and grades, such as, for example, MFC and film coated LWC as well as SC-C can be produced by means of it.
Multi-roll on-line, off-line calendering is calendering in a calender unit in which the number of rolls is higher than in soft calenders, most commonly 6-16. Multi-roll calenders are soft-nip calenders. The resilient-surface roll conforms to the contours of the surface of paper and presses the opposite side of paper evenly against the smooth-surface press roll. Linear load increases in the multi-roll calender from the top nip to the bottom nip because of the force of gravity. By using the relieving of rolls, this increase in load can be compensated for. This kind of relieving of the rolls is provided in Metso Paper, Inc.""s OptiLoad(trademark) calender. Multi-roll on-line, off-line calendering is a calendering method, allowing grades from WFS up to uncoated fine paper to be produced.
The primary object of the present invention is
to improve calendering of a fibrous web in connection with a papermaking process,
to improve control of the moisture gradient of a fibrous web, such as a paper or board web,
to diminish the process problems now associated with the manufacture of high quality paper grades, such as WFC, LWC-roto and SC-A, and
to enable the manufacture of high quality paper grades, such as WFC, LWC-roto and SC-A by on- or off-line calendering.
Thus, the invention is based on the new and inventive idea that an on- or off-line multi-roll calender comprising separate sets of rolls is used for calendering, and that the fibrous web is subjected to intermediate moistening between the sets of rolls. In accordance with an advantageous embodiment of the invention, the multi-roll calender comprises two sets of rolls, in which connection the moisture content of the fibrous web coming from the drying process is raised to a level of 3-10% by means of pre-moistening preceding the first set of rolls, the fibrous web is dried to a level of 1-6% in the first set of rolls, the moisture content of the fibrous web is raised to a level of 6-14% by means of intermediate moistening after the first set of rolls, and the fibrous web is dried in the second set of rolls to a desired final moisture level, which is advantageously in a range of 4.5-7.5%.
With respect to the benefits of the invention, it may be mentioned that by means of the multi-stage moistening and gradient calendering according to the invention it is possible to better and more accurately affect only the surface layers of the fibrous web and to leave the inner layers of the fibrous web substantially untouched, which allows higher quality paper grades to be produced by on- or off-line calendering.